The present invention relates to the field of polymer films and, more particularly, to a uniaxially heat shrinkable biaxially oriented polypropylene film.
As noted in U.S. Pat. No. 4,194,039, polyolefins can be used to prepare shrink films for wrapping purposes. Other suitable synthetic resins include various ionomers, polyvinyl chlorides, polyesters, polystyrenes and polyvinylidene chlorides.
A shrink film's distinguishing characteristic is its ability upon exposure to some level of heat to shrink or, if restrained, to create shrink tension within the film. This ability is activated by the packager when the wrapped product is passed through a hot air or hot water shrink tunnel. The resulting shrinkage of the film results in an aesthetically pleasing transparent wrapping which conforms to the contour of the product while providing the usual functions required of packaging materials such as protection of the product from loss of components, pilferage, or damage due to handling and shipment. Typical items wrapped in polyolefin shrink films are toys, games, sporting goods, stationery, greeting cards, hardware and household products, office supplies and forms, foods, phonograph records, and industrial parts.
In certain situations it is desirable to effect shrinkage along a single axis without substantial shrinkage in the cross-direction. For example, in the process of labeling bottles or cans by shrinking a tube or lap-sealed sleeve of heat shrinkable material, if the film shrinks parallel to the container axis, the label may not be placed in the right position but rather placed above or below the desired position upon shrinkage.
In order to obtain uniaxially shrinkable materials, it is possible to employ uniaxially oriented materials, i.e., materials which are oriented in only one direction. However, uniaxially oriented film can lack the requisite strength and toughness necessary for use in such applications. Inasmuch as biaxially oriented films exhibit desirable strength and tear resistance in both directions of orientation, it would be desirable to obtain a uniaxially heat shrinkable film which is biaxially oriented but substantially stable in the cross-direction. In labeling applications, the shrinkable direction usually corresponds to the machine direction (MD) of the film manufacturing process.
For more detailed disclosures of heat shrinkable films, reference may be had to aforesaid U.S. Pat. No. 4,194,039, as well as U.S. Pat. Nos. 3,808,304; 4,188,350; 4,377,616; 4,390,385; 4,448,792; 4,582,752 and 4,963,418, all of which are incorporated herein by reference.
U.S. Pat. No. 5,292,561 (corresponding to EPA 0498249) discloses a process for producing polyolefin shrink films having high unidirectional shrinkage (at least 10% longitudinal shrinkage and less than 2% transverse shrinkage at 100.degree. C.) under conditions comprising an MD reorientation mechanical MD/TD draw ratio between 1.01 and 7.5. The base layer of the films contain propylene polymer and optionally, hydrogenated hydrocarbon resin.
EPA 0204843 discloses a low temperature shrinkable film comprising linear low-density polyethylene resin having film shrink properties of 30% or more MD and 5% or less TD at 90.degree. C., which is prepared by drawing the film at a high draw ratio (3 to 6) in the machine direction.
EPA 0321964 describes a process for extruding a shrink film from a linear low density copolymer of ethylene and at least one alpha-olefin having 3 to 6 carbon atoms to provide a material which exhibits shrinkage at 135.degree. C. of at least 30% MD and at least 10% TD.
EPA 0477742 discloses a transparent polypropylene shrink film which exhibits shrinkage at 100.degree. C. of at least 10% MD and less than 2% TD. The polypropylene comprises a 15% or less, preferably 2 to 6% n-heptane soluble component.
EPA 0299750 discloses a mono- or biaxially stretched film having a heat shrinkage of 20% or more in one of the longitudinal and transverse directions and 60% or more in the other direction. The film comprises principally a linear polyethylene and optionally, a branched low-density polyethylene.
EPA 0595270 discloses a heat sealable laminate having high unidirectional shrinkage produced from biaxially oriented polymeric film such as biaxially oriented polypropylene or blends of polypropylene and copolymers of propylene with minor amounts of ethylene or an alpha-olefin. Uniaxial shrinkability is achieved by balancing MD reorientation process variables such as temperature, draw ratio, line speed, and oriented polymer film properties. Heat sealability is imparted by the presence of a heat seal layer.
Generally, the desired high level of MD shrinkage at label application temperatures (above about 130.degree. C.) is accompanied by proportionally high MD shrinkage at storage temperatures. High MD shrinkage at storage temperatures results in roll tightening and ensuing film distortions and blocking. Another consequence of high MD shrinkage at storage temperatures in the case of film stored after printing is loss of accuracy in the MD label repeat distance.
It is the object of the present invention to provide a biaxially oriented multilayer film comprising a polypropylene core which is uniaxially heat shrinkable with minimal storage temperature shrinkage.